Optical circulator

ABSTRACT

An improved optical circulator transfers light from a first optical port to a second optical port, and from the second port to a third port. The circulator has non-reciprocal polarization rotators, birefringent beam splitters and combiners, and a polarization-dependent light bending device comprising two tapered birefringent plates. The light bending device compensates for an angle between a first light beam emanating from the first port and a second light beam propagating to the third port. The existence of this angle allows the first and third fibers to be coupled to the light beams using a single lens.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part of patent applicationSer. No. 09/025,526 filed on Feb. 18, 1998. patent application Ser. No.09/025,526 is a continuation-in-part of patent application Ser. No.08/986,064 filed Dec. 5, 1997.

FIELD OF THE INVENTION

[0002] This invention relates to non-reciprocal couplings for opticalfibers, and in particular, to optical circulators.

BACKGROUND

[0003] An optical circulator is a device that has at least three portsfor accepting optical fibers. Light that enters the circulator throughthe first port exits through the second port; light that enters throughthe second port exits through the third. The optical circulator is aninherently non-reciprocal device, since if light enters through thefirst port it exits through the second, but if that light issubsequently reflected back into the second port, it does not retraceits path back to the first port, but exits through the third portinstead.

[0004] Circulators are necessary, for example, to use the same fiber forboth receiving and transmitting data. The first port may be connected toa data transmitter, and the second port to a long distance opticalfiber. In that case, data can be sent from the transmitter to the fiber.At the same time, incoming optical data from the long distance fiberenters the circulator through the second port and is directed to thethird port where a receiver may be connected.

[0005] One prior art optical circulator is described in U.S. Pat. No.4,650,289 by Kuwahara; see FIG. 1. In this circulator, the labels A, B,and C correspond to the first, second, and third ports described above(port D need not be used). This circulator suffers from the followingdisadvantages: it requires two spatially separated optical paths, andthe ports A and C are perpendicular. This means that the circulator willbe bulky when a more compact size is desirable.

[0006] A more compact circulator is described in U.S. Pat. No. 5,204,771by Koga; see FIG. 2. This circulator shows an improvement over theprevious one since the two optical paths can be in close proximity, andthe first and third ports (designated 27 and 28 in the drawing) areparallel. Unfortunately, this device still suffers from a disadvantage.A lens must be placed between the first optical fiber and the circulatorto collimate light coming from the first fiber. A lens must also beplaced between the third fiber and the circulator to focus light ontothe third fiber. If the first and third fibers are far enough apart thatthere is room to insert two lenses side by side (one for each fiber),the circulator will have to be quite large. Such a circulator will alsobe expensive, since the cost increases with the size of the components.

[0007] If the first and third ports (27 and 28 in FIG. 2) are very closetogether, the first and third fibers will have to share a common lensfor collimating and focusing. However, it is impossible for a singlelens to perform adequately for both fibers. The difficulty can be tracedto the fact that the light beams coupled to the first and third portsare parallel, and a single lens cannot focus two parallel beams to twodifferent points (i.e., to two different fibers). This prior arttherefore suffers from the shortcoming that it cannot be manufacturedeconomically when the circulator is large, and it cannot be efficientlycoupled to optical fibers when the circulator is small.

OBJECTS AND SUMMARY OF THE INVENTION

[0008] In view of the above, it is an object of the present invention toprovide a compact and economical optical circulator that can beefficiently coupled to optical fibers.

[0009] The invention consists of an optical circulator having at leastthree ports for optical fibers. Light beams coupled to the first andthird fibers are not parallel; there is a slight angle between the twobeams. Because of this angle, a single lens may be used for couplingboth the first and the third fibers to the circulator.

[0010] The invention further consists of a light-bending devicecomprising two tapered birefringent plates, situated to compensate forthe angle between the light beams coupled to the first and third fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows a prior art optical circulator by Kuwahara.

[0012]FIG. 2 shows a prior art optical circulator by Koga.

[0013]FIG. 3 shows how light is transmitted from a first optical fiberto a second optical fiber in a first embodiment of a circulatoraccording to the invention.

[0014]FIG. 4 shows how light is transmitted from the second opticalfiber to a third optical fiber in the circulator of FIG. 4.

[0015]FIG. 5a shows various embodiments of a polarization-dependentlight guiding device when n_(o)>n_(e.)

[0016]FIG. 5b shows various embodiments of the polarization-dependentlight guiding device when n_(o)<n_(e.)

[0017]FIG. 6 shows a three dimensional view of a second embodiment ofthe circulator with a light beam propagating from a first fiber to asecond fiber.

[0018]FIG. 7a is a top plan view of the circulator of FIG. 6 showing alight beam propagating from the first fiber to the second fiber.

[0019]FIG. 7b is a side view of the circulator of FIG. 6 showing thelight beam propagating from the first fiber to the second fiber.

[0020]FIG. 8a is a top plan view of the circulator of FIG. 6 showing alight beam propagating from the second fiber to the third fiber.

[0021]FIG. 8b is a side view of the circulator of FIG. 6 showing thelight beam propagating from the second fiber to the third fiber.

[0022]FIG. 9 shows a three dimensional view of a third embodiment of thecirculator with a light beam propagating from the first fiber to thesecond fiber.

[0023]FIG. 10a is a top plan view of a fourth embodiment of thecirculator with a light beam propagating from the first fiber to thesecond fiber.

[0024]FIG. 10b is a side view of the circulator of FIG. 10a showing thelight beam propagating from the first fiber to the second fiber.

[0025]FIG. 11a is a top plan view of the circulator of FIG. 10a showinga light beam propagating from the second fiber to the third fiber.

[0026]FIG. 11b is a side view of the circulator of FIG. 10a showing thelight beam propagating from the second fiber to the third fiber.

[0027]FIG. 12a is a top plan view of a fifth embodiment of thecirculator with a light beam propagating from the second fiber to thethird fiber.

[0028]FIG. 12b is a side view of the circulator of FIG. 12a showing thelight beam propagating from the second fiber to the third fiber.

DETAILED DESCRIPTION OF THE INVENTION

[0029]FIG. 3 shows a circulator 100 according to a first embodiment ofthe invention. A first optical fiber 1 is inserted into a first glasscapillary 10A. A second optical fiber 2 is inserted into a second glasscapillary 10B opposite first fiber 1. A third optical fiber 3 isinserted into first glass capillary 10A adjacent to fiber 1, so thatfiber 3 and fiber 1 are parallel.

[0030] A set of orthogonal reference axes is arranged so that the y-axisis parallel to fibers 1, 2, and 3. A reference point P is located nearthe first glass capillary 10A.

[0031] Fiber 1 emits a light beam 30 that is collimated by a first lens12A. Lens 12A also causes beam 30 to make an angle θ with respect to they-axis. Preferably, lens 12A is a graded index (GRIN) lens.

[0032] Beam 30 then passes through a first birefringent block 14A. Beam30 is thereby divided into two beams having orthogonal polarizations,specifically beams 30A and 30B, corresponding to the ordinary andextraordinary rays in birefringent block 14A. Beam 30A is polarizedalong the x-axis (out of the page); this polarization is indicated by adot in FIG. 3. Beam 30B is polarized in the y-z plane; this polarizationis indicated by a line segment. The length of birefringent block 14A isadjusted to obtain a spatial separation between beams 30A and 30B whichpermits to pass them through independent optical elements.

[0033] Thus, beam 30A enters a first half wave plate 18A which rotatesthe plane of polarization by 45° in the counterclockwise direction asseen from point P in FIG. 3. Beam 30A then enters a first Faradayrotator 20A which rotates the plane of polarization by 45° in theclockwise direction as seen from point P. The net effect of half waveplate 18A and Faraday rotator 20A (the first a reciprocal device and thesecond non-reciprocal), therefore, is to leave the polarization of beam30A unaltered.

[0034] Beam 30B, meanwhile, enters a second half wave plate 16Apositioned above first half wave plate 18A. Second half wave plate 16Arotates the plane of polarization of beam 30B by 45° clockwise as seenfrom P; i.e., half wave plate 16A effects a rotation in the oppositedirection to half wave plate 18A. Beam 30B then passes through a Faradayrotator 20A, which again rotates the plane of polarization by 45°clockwise as seen from P. Therefore, after passing through half waveplate 16A and Faraday rotator 20A, the polarization of beam 30B is inthe x-direction, or parallel to the polarization of beam 30A.

[0035] Half wave plates 16A and 18A, together with Faraday rotator 20A,make up a first compound polarization rotator 40A that renders twoorthogonal polarizations parallel to each other.

[0036] At this point beams 30A and 30B still propagate at angle θ withrespect to the y-axis as they exit rotator 20A. This angle ofpropagation is changed by a polarization-dependent light guiding device42. Device 42 consists of a first tapered birefringent plate 22 and asecond tapered birefringent plate 24. The tapering of plate 22 iscomplementary to the tapering of plate 24, and each plate is tapered byan angle α. Plates 22 and 24 are made from the same birefringentmaterial and each plate has two indices of refraction: n_(e) and n_(o),corresponding to the extraordinary and ordinary rays. In the embodimentillustrated in FIG. 3, n_(o)>n_(e.)

[0037] The index of refraction in general determines how much a lightray will bend, or refract, upon entering a material. When the index ofrefraction is known, the amount of refraction can be determined bySnell's law. A birefringent material has two indices of refraction,indicating that different polarizations of light will refract bydifferent amounts.

[0038] Plate 22 has an optic axis OA1 that is oriented parallel to thex-axis. Therefore beams 30A and 30B are viewed as extraordinary rays inplate 22, and are therefore refracted according to the extraordinaryindex of refraction n_(e). Plate 24 has an optic axis OA2 that isparallel to the z-axis, so beams 30A and 30B are ordinary rays withinplate 24. Therefore beams 30A and 30B are refracted upon passing fromplate 22 to plate 24 because of the difference between indices ofrefraction n_(e) and n_(o.)

[0039] The angle α is adjusted so that beams 30A and 30B are renderedparallel to the y-axis by light guiding device 42. Using Snell's law ateach interface, the relationship between the angles α and θ is:

sin θ=n _(e) sin{sin⁻¹[(n _(o) /n _(e))sin α]−α}.  (1)

[0040] Beams 30A and 30B exit plate 24 and enter a second birefringentblock 26. The optical axis of block 26 is oriented such that beams 30Aand 30B are ordinary rays in block 26 and thus remain undeflected.

[0041] Next, beam 30A enters a half wave plate 18B which rotates theplane of polarization of beam 30A by 45° counterclockwise as seen frompoint P. Beam 30A then passes through a Faraday rotator 20B whichrotates the polarization by another 45° counterclockwise as seen from P.Beam 30A is now polarized in the z-direction (indicated by a linesegment in the figure).

[0042] Meanwhile, beam 30B passes through a half wave plate 16B whichrotates the polarization 45° clockwise as seen from P. Beam 30B thenenters Faraday rotator 20B which rotates the plane of polarization ofbeam 30B by 45° counterclockwise as seen from point P. Consequently,half wave plate 16B and rotator 20B together have no net effect on thepolarization of beam 30B.

[0043] Half wave plates 16B and 18B and Faraday rotator 20B togethercomprise a second compound polarization rotator 40B that renders twoparallel polarizations perpendicular to each other.

[0044] Beams 30A and 30B subsequently pass through a third birefringentblock 14B, where beam 30A is the extraordinary ray and beam 30B isordinary. Block 14B combines beams 30A and 30B to form a single beam 31that is in general unpolarized since it combines the two orthogonalpolarizations of beams 30A and 30B.

[0045] Beam 31 is focused by a second lens 12B (preferably a GRIN lens)and enters optical fiber 2 mounted in glass capillary 10B.

[0046] The description so far shows how light starting from fiber 1 isguided into fiber 2. For circulator 100 to work properly, light enteringthe circulator from fiber 2 must be channeled into fiber 3. In otherwords, circulator 100 has the property of channeling light from fiber 1to fiber 2 and from fiber 2 to fiber 3 without any light being channeledfrom fiber 2 back to fiber 1. This second step is shown in FIG. 4.

[0047] Thus, a beam 32 exits fiber 2 and is collimated by lens 12B. Beam32 then enters birefringent block 14B and is split into two beams, 32Aand 32B, having orthogonal polarizations. Beam 32A is ordinary, beam 32Bextraordinary in block 14B. Upon leaving block 14B, beam 32A ispolarized in the x-direction and beam 32B is polarized in they-direction, as indicated in FIG. 4.

[0048] Beams 32A and 32B next enter compound polarization rotator 40B.Beam 32A enters Faraday rotator 20B, which rotates the polarization ofbeam 32A by 45° counterclockwise as seen from point P. Then beam 32Aenters half wave plate 16B, which rotates the polarization of beam 32Aby another 45° counterclockwise as seen from P.

[0049] Meanwhile the polarization of beam 32B is rotated by 45°counterclockwise as seen from point P by Faraday rotator 20B. Thepolarization of beam 32B is then rotated back 45° clockwise as seen fromP by half wave plate 18B.

[0050] Therefore, just before beams 32A and 32B enter birefringent block26, they are both polarized in the z-direction. Here the non-reciprocalnature of circulator 100 is already clear, since if beams 32A and 32Bwere to exactly retrace the paths of beams 30A and 30B (FIG. 3), theyshould be polarized in the x-direction. The origin of thenon-reciprocity is the Faraday rotator 20B, whose direction ofpolarization rotation does not reverse with the change in direction oflight propagation.

[0051] Beams 32A and 32B enter birefringent block 26, where they areextraordinary rays and are offset by a distance f. Both beams 32A, 32Bthen enter light guiding device 42, which causes beams 32A and 32B toeach make an angle φ with respect to the y-axis.

[0052] Beams 32A and 32B now enter birefringent plate 24. The optic axisOA2 of plate 24 is parallel to the polarizations of beams 32A and 32B.Beams 32A and 32B are therefore extraordinary rays within plate 24, butare undeflected since they are normally incident upon plate 24.

[0053] Upon leaving plate 24 and entering plate 22, however, beams 32Aand 32B become ordinary rays, since their polarizations areperpendicular to the optic axis OA1 of plate 22. Beams 32A and 32Btherefore refract upon entering plate 22 due to the difference betweenrefraction indices n_(e) and n_(o). When beams 32A and 32B exit plate22, they refract again to exit at angle φ with respect to the y-axis.Using Snell's law, the relationship between angle φ and angle α is asfollows:

sin φ=n _(o) sin{α−sin⁻¹[(n _(e) /n _(o))sin α]}.  (2)

[0054] After leaving light guiding device 42, beam 32A then passesthrough Faraday rotator 20A and half wave plate 16A with no net effecton its polarization. Beam 32B passes through Faraday rotator 20A andhalf wave plate 18A; the result is a rotation of the polarization ofbeam 32A by 90° clockwise as seen from point P. Beams 32A and 32B nowhave orthogonal polarizations and are combined into a single beam 33 bybirefringent block 14A. Beam 33 is subsequently focused by lens 12A ontofiber 3.

[0055] Birefringent block 26 is a polarization-dependent beam deflectorthat offsets beams 32A and 32B but does not offset beams 30A and 30B.Birefringent block 26 plays an important role in guiding light fromfiber 2 to fiber 3. Since light guiding device 42 bends beams 32A and32B by the angle φ, beams 32A and 32B travel laterally (in the negativez-direction) as well as longitudinally (in the negative y-direction)after they leave device 42. This lateral travel is compensated by block26.

[0056] To be precise, beams 32A and 32B are offset a distance f bybirefringent block 26. The distance along the z-axis between the pointwhere beam 32B enters plate 24 and fiber 3 is d₂ (see FIG. 4). Thedistance along the z-axis between fiber 1 and the point where beam 30Aleaves plate 24 is d₁ (see FIG. 3). The vertical or z-axis distancebetween fiber 1 and fiber 3 is t. The relation between these quantitiesis:

f=d ₁ +d ₂ −t.  (3)

[0057] This equation teaches how to design block 26 to have the correctoffset f given the other parameters of circulator 100, i.e., when d₁,d₂, and t are known.

[0058] In an alternative embodiment, the apparatus is designed in such away that d₁+d₂=t. Eq. (3) then implies that f=0, which means thatbirefringent block 26 can be eliminated completely from the design.

[0059] If angle θ were exactly equal to angle φ, fibers 1 and 3 would beplaced symmetrically with respect to the center of lens 12A. That is,the lateral distance (distance measured along the z-axis) from fiber 1to the center of lens 12A would equal the lateral distance from fiber 3to the center of lens 12A. However, angles θ and φ are onlyapproximately equal: if angles θ, φ, and α are all small, then equations(1) and (2) yield to a first approximation:

θ≈(n _(o) −n _(e))α≈φ

[0060] To a better approximation, angle θ differs slightly from angle φ.This difference can be accommodated in at least two ways. The firstoption is to adjust the lateral positions (i.e. z-coordinates) of fibers1 and 3 so that the fibers are asymmetric with respect to the center oflens 12A. The second, preferred option is to place fibers 1 and 3symmetrically with respect to the center of lens 12A, and to rotatelight guiding device 42 slightly about an axis parallel to the x-axis,thereby altering equations (1) and (2) to ensure that θ=φ. Eitherapproach represents a minor adjustment of the overall apparatus. Inpractice, angles φ and θ are between 1° and 3°, and light guiding device42 is rotated a fraction of a degree.

[0061] Birefringent elements 14A, 14B, 22, 24, and 26 can be made of anybirefringent material, such as rutile, calcite, or yttriumorthovanadate.

[0062] It should be clear that several variations of the aboveembodiment are possible and remain within the scope of the invention.For example, the polarizations of beams 30A and 30B need not be exactlyas shown. It is only important that the polarizations of beams 30A and30B are orthogonal or perpendicular to each other when the beams exitblock 14A, and that the polarizations are parallel after leaving rotator20A. When the polarizations of beams 30A and 30B are not as describedabove, the optic axes of the birefringent elements 14A, 14B, 22, 24, and26 are adjusted accordingly. This adjustment changes the polarizationsof beams 32A and 32B. However, as is apparent to a person of averageskill in the art, the principles of circulator 100 remain unchanged.

[0063] Thus, in another embodiment, beam 30A is extraordinary and beam30B is ordinary in block 14A. In this embodiment, the beams havecomplementary properties in block 14B: beam 30A is ordinary and beam 30Bis extraordinary. This arrangement ensures, as does the embodiment ofcirculator 100, that beams 30A and 30B both traverse approximately thesame optical path, and therefore the overall phase relation between themis maintained.

[0064] Variations of light guiding device 42 are also possible. FIG. 5ashows different shapes and orientations of optic axes OA1 and OA2 thatplates 22 and 24 can have when n_(o)>n_(e). If plates 22 and 24 are madeof some birefringent material with n_(o)<n_(e), other geometries areused, as shown in FIG. 5b. Still other variations are possible: in theexamples of FIG. 5a and FIG. 5b, plates 22 and 24 each have one faceparallel to the z-axis. However, a more general trapezoidal shape can beused for either or both of plates 22 and 24, with no faces parallel tothe z-axis. Furthermore, plate 22 need not be made of the same materialas plate 24.

[0065] In a second embodiment a circulator 200 is designed such thatangles θ and φ lie in the same plane while the walk-off in thebirefringent blocks takes place in a perpendicular plane. The generalconstruction and operation of this embodiment is analogous to that ofcirculator 100 and is illustrated in the three dimensional view of FIG.6.

[0066] First and third fibers 202, 204 are inserted in parallel andadjacent to each other into a glass capillary 206A which is followed bya first lens 208A. A first block of birefringent material 210A, a firstcompound polarization rotator 230A, a light guiding device 250comprising first and second tapered birefringent plates 252 and 254, asecond birefringent block 256, a second compound polarization rotator230B and a third block of birefringent material 210B are located along alongitudinal axis L of circulator 200. A second lens 208B and a secondglass capillary 206B holding a second fiber 258 are found at theopposite end of device 200.

[0067] Longitudinal axis L is parallel to the y-axis. In distinction tocirculator 100 where first and third fibers 1, 3 are inserted one belowthe other (along the z-axis) fibers 202, 204 are arranged next to eachother (along the x-axis).

[0068] In circulator 200, first compound polarization rotator 230Acomprises first and second half-wave plates 220A and 222A, a n d a firstFaraday rotator 224A. Second compound polarization rotator 230Bcomprises third and fourth half-wave plates 220B and 222B, and a secondFaraday rotator 224B.

[0069] A first light beam 240 propagating from first fiber 202 entersfirst block 210A and the two orthogonal polarizations 240A and 240B arewalked off within block 210A as shown. These polarizations continuepropagating through the elements of circulator 200 until they arerecombined by third block 210B and focused by second lens 208B intosecond fiber 258.

[0070] The top view of FIG. 7a also shows first light beam 240propagating from fiber 202 to fiber 258 through the elements ofcirculator 200. Angle θ, made by beam 240 with respect to longitudinalaxis L when exiting through first lens 208A, lies in the x-y plane.Meanwhile, as shown in the side view of FIG. 7b, the walk off of the twoorthogonal polarizations 240A and 240B in birefringent block 210A occursin the y-z plane.

[0071] When a second light beam 270 propagates from second fiber 258 tothird fiber 204, as illustrated in FIGS. 8a-b, it is offset by distancef in second block 256. Note that offset distance f is in the x-y plane(FIG. 8a). Next, in light guiding device 250 beam 270 is bent at angle φwith respect to longitudinal axis L. In other words, beam 270 exitslight guiding device 250 at angle φ. Angle φ also lies in the x-y plane.Thus, angles φ and θ lie in planes which are parallel while the walk-offoccurs in a plane perpendicular to them.

[0072] The advantage of having angles φ and θ lie in an x-y plane whilethe walk-off takes place in the y-z plane is that it is easier to adjustangles φ and θ independently of the walk-off. Specifically, in practiceit is easier to adjust the positions of the elements of circulator 200to obtain proper coupling of beams 240 and 270 between fibers 202, 258and 204 when the walk-off and the compensating angles φ, θ are inperpendicular planes. Also, in this configuration the elements ofcirculator 200 can be made smaller and the entire circulator is easierto manufacture.

[0073] Because circulators 100 and 200 comprise half-wave plates, theefficiencies of circulators 100 and 200 are sensitive to the wavelengthof light transmitted. A circulator 300 according to a third embodimentis shown in FIG. 9. Circulator 300 is nearly identical to circulator 200except that first and second compound polarization rotators 330A and330B of circulator 300 comprise only non-reciprocal elements. Circulator300 is preferred over circulator 200 because circulator 300 isinsensitive to the wavelength of light used, and has fewer parts.

[0074] In FIG. 9, light beam 240 emerges from first fiber 202 and entersa first birefringent block 310A. Beam 240 then diverges into two beams301 and 302 corresponding to the ordinary and extraordinary rays inblock 310A. Beams 301 and 302 have orthogonal polarizations 340A and340B, respectively, in block 310A. Block 310A has an optic axis along adirection such that polarizations 340A and 340B each make a 45° anglewith the z-axis.

[0075] First compound polarization rotator 330A comprises a firstFaraday rotator 320A and a second Faraday rotator 322A. Faraday rotator320A rotates polarization 340B by 45° clockwise. Faraday rotator 322Arotates polarization 340A by 45° counter-clockwise. Therefore, beams 301and 302 emerge from compound polarization rotator 330A withpolarizations parallel to the z-axis, as shown in FIG. 9.

[0076] Beams 301 and 302 then propagate through light guiding device 250and second birefringent block 256 just as in circulator 200. Beams 301and 302 then reach second compound polarization rotator 330B. Compoundpolarization rotator 330B comprises a third Faraday rotator 320B and afourth Faraday rotator 322B. Faraday rotator 320B rotates thepolarization of beam 301 by 45° clockwise, and Faraday rotator 322Brotates the polarization of beam 302 by 45° counter-clockwise.

[0077] Beams 301 and 302 therefore emerge from compound polarizationrotator 330B with polarizations 340D and 340C, respectively.Polarizations 340C and 340D are orthogonal, and each makes a 45° anglewith respect to the z-axis. Beams 301 and 302 are subsequentlyrecombined by a birefringent block 310B, and focused by lens 208B ontosecond fiber 258.

[0078] When light is emitted from second fiber 258 in circulator 300,the light is split into polarizations 340C and 340D by block 310B.Polarizations 340C and 340D are then rendered parallel to the x-axis bycompound polarization rotator 330B. The light is then guided into thirdfiber 204 according to the principles outlined above.

[0079] A circulator 400 according to a fourth embodiment is shown inFIGS. 10a and 10 b. Circulator 400 is identical to circulator 200 withthe following exceptions: birefringent block 256 is omitted, and lightguiding device 250 is replaced by a light guiding device 450.

[0080] Light guiding device 450 comprises first and second taperedbirefringent plates 452 and 454. Tapered plate 452 has an optic axis OA3parallel to the z-axis; tapered plate 454 has an optic axis OA4 thatlies in the xy-plane, as shown in FIG. 10a. Optic axis OA4 is neitherparallel to nor perpendicular to longitudinal axis L. That is, opticaxis OA4 is skewed with respect to longitudinal axis L.

[0081] When beam 240 emerges from first fiber 202, block 210A dividesbeam 240 into two beams 401 and 402 having orthogonal polarizations; seeFIG. 10b. Upon exiting first compound polarization rotator 230A, beams401 and 402 have polarizations 440 parallel to the z-axis. Beams 401 and402 then propagate through light guiding device 450 to enter secondfiber 258 as in circulator 200.

[0082] When beam 270 is emitted from second fiber 258, block 210Bdivides beam 270 into two beams 403 and 404 having orthogonalpolarizations, as shown in FIGS. 11a and 11 b. Compound polarizationrotator 230B causes beams 403 and 404 to have polarizations 441 parallelto the x-axis.

[0083] When beams 403 and 404 enter tapered plate 454, the beams areoffset in the x-direction by a distance f′, as shown in FIG. 11a. Thisoffset occurs because optic axis OA4 is neither perpendicular to norparallel to polarizations 441 of beams 403 and 404. In circulator 400,therefore, tapered plate 454 performs the functions of both birefringentblock 256 and tapered plate 254 of circulator 200.

[0084] Beams 403 and 404 are offset by tapered plate 454, and are thenrefracted by tapered plate 452. Beams 403 and 404 exit tapered plate 452at an angle φ with respect to the longitudinal axis L, as shown in FIG.11a. Beams 403 and 404 are then directed to third fiber 204, as incirculator 200.

[0085] The exact angle that optic axis OA4 makes with respect tolongitudinal axis L, as well as the precise length (in the y-direction)of tapered plate 454, can be easily determined by one skilled in theart.

[0086] Circulator 400 is preferred over circulator 200 since circulator400 eliminates the need for birefringent block 256. Relative tocirculator 200, circulator 400 is smaller, cheaper, and has a lowerlight loss from reflections off the surfaces of components.

[0087] A circulator 500 according to a fifth embodiment is shown inFIGS. 12a and 12 b. In the fifth embodiment, compound polarizationdevices 230A and 230B of circulator 400 are replaced by compoundpolarization devices 330A and 330B of circulator 300. Accordingly,blocks 210A and 210B are replaced by blocks 310A and 310B. The fifthembodiment combines the advantages of both circulators 300 and 400: thefifth embodiment comprises compound polarization rotators having Faradayrotators but not half-wave plates, and the fifth embodiment does notcontain birefringent block 256.

[0088] When beam 270 is emitted from second fiber 258 of circulator 500,beam 270 is divided into beams 503 and 504 by birefringent block 310B,as shown in FIGS. 12a and 12 b. Beams 503 and 504 enter compoundpolarization rotator 330B comprising Faraday rotators 320B and 322B.Upon exiting compound polarization rotator 330B, beams 503 and 504 havepolarizations 541 parallel to the x-axis. Beams 503 and 504 then enterthird fiber 204 after passing through light guiding device 450, compoundpolarization rotator 330A, birefringent block 310A, and lens 208A.

[0089] Many variations of circulator 500 are possible and remain withinthe scope of the invention. For example, the directions of polarizations540 and 541 may be changed if the directions of optic axes OA3 and OA4are correspondingly altered. Furthermore, the shapes of tapered plates452 and 454 are subject to the same variation as shown in FIG. 5 fortapered plates 22 and 24.

[0090] In the broad sense, the circulator can be used to couple lightbetween three optical ports. The ports can include optical fibers as inthe embodiments above or other optical elements.

[0091] It will be apparent to a person of average skill in the art thatmany variations of the circulator are possible within the scope of theinvention. Accordingly, the scope of the invention should be determinedby the following claims and their legal equivalents.

We claim:
 1. An optical circulator for coupling light from a first fiberto a second fiber and from said second fiber to a third fiber, saidfirst fiber and said third fiber being located adjacent to each otheralong a longitudinal axis, and said second fiber being located oppositesaid first and third fibers along said longitudinal axis, said opticalcirculator comprising along said longitudinal axis in sequence from saidfirst fiber to said second fiber: a) a first lens for guiding light fromsaid first fiber and to said third fiber; b) a first block ofbirefringent material for separating and combining mutually orthogonalpolarizations; c) a first compound polarization rotator for renderingmutually parallel polarizations orthogonal and mutually orthogonalpolarizations parallel, said first compound polarization rotatorcomprising first and second non-reciprocal polarization rotators; d) apolarization-dependent light guiding device, comprising a first and asecond tapered plates of birefringent material, said first plate havinga first optic axis, said second plate having a second optic axis, saidfirst optic axis and said second optic axis being mutuallyperpendicular; e) a polarization-dependent beam deflector comprising asecond block of birefringent material, f) a second compound polarizationrotator for rendering mutually parallel polarizations orthogonal andmutually orthogonal polarizations parallel, said second compoundpolarization rotator comprising third and fourth non-reciprocalpolarization rotators; g) a third block of birefringent material forseparating and combining mutually orthogonal polarizations; and h) asecond lens for guiding light to said second fiber and from said secondfiber, wherein light passing from said first fiber exits said first lensat an angle θ with respect to said longitudinal axis, and said lightpassing from said second fiber exits said light guiding device at anangle φ with respect to said longitudinal axis.
 2. The opticalcirculator of claim 1 wherein said first and second lenses are gradedindex lenses.
 3. The optical circulator of claim 1 wherein said angle θis between 1° and 3°, and said angle φ is between 1° and 3°.
 4. Theoptical circulator of claim 1 wherein said polarization-dependent lightguiding device is rotated such that said angle φ is substantially equalto said angle θ.
 5. The optical circulator of claim 1 wherein saidfirst, second, and third blocks of birefringent material and said firstand second tapered plates comprise a material selected from the groupconsisting of rutile, calcite, and yttrium orthovanadate.
 6. The opticalcirculator of claim 1 wherein said polarization-dependent beam deflectoroffsets light traveling from said second port by a distance fperpendicular to said longitudinal axis.
 7. The optical circulator ofclaim 6 wherein said distance f is in the plane of said angle φ.
 8. Theoptical circulator of claim 1 wherein said angle θ lies in a first planeand said angle lies in a second plane parallel to said first plane. 9.The optical circulator of claim 8 wherein said mutually orthogonalpolarizations and said mutually parallel polarizations are separated andcombined in said first block of birefringent material and in said secondblock of birefringent material in a third plane perpendicular to saidfirst plane and said second plane.
 10. An optical circulator forcoupling light from a first fiber to a second fiber and from said secondfiber to a third fiber, said first fiber and said third fiber beinglocated adjacent to each other along a longitudinal axis, and saidsecond fiber being located opposite said first and third fibers alongsaid longitudinal axis, said optical circulator comprising along saidlongitudinal axis in sequence from said first fiber to said secondfiber: a) a first lens for guiding light from said first fiber and tosaid third fiber; b) a first block of birefringent material forseparating and combining mutually orthogonal polarizations; c) a firstcompound polarization rotator for rendering mutually parallelpolarizations orthogonal and mutually orthogonal polarizations parallel;d) a polarization-dependent light guiding device, comprising a first anda second tapered plates of birefringent material, said first platehaving a first optic axis, said second plate having a second optic axis,said first optic axis and said second optic axis being mutuallyperpendicular, and said second optic axis being skewed with respect tosaid longitudinal axis; e) a second compound polarization rotator forrendering mutually parallel polarizations orthogonal and mutuallyorthogonal polarizations parallel; f) a second block of birefringentmaterial for separating and combining mutually orthogonal polarizations;and g) a second lens for guiding light to said second fiber and fromsaid second fiber, wherein light passing from said first fiber exitssaid first lens at an angle θ with respect to said longitudinal axis,and said light passing from said second fiber exits said light guidingdevice at an angle φ with respect to said longitudinal axis.
 11. Theoptical circulator of claim 10 wherein said first and second lenses aregraded index lenses.
 12. The optical circulator of claim 10 wherein saidangle θ is between 1° and 3°, and said angle φ is between 1° and 3°. 13.The optical circulator of claim 10 wherein said polarization-dependentlight guiding device is rotated such that said angle φ is substantiallyequal to said angle θ.
 14. The optical circulator of claim 10 whereinsaid first and second blocks of birefringent material and said first andsecond tapered plates comprise a material selected from the groupconsisting of rutile, calcite, and yttrium orthovanadate.
 15. Theoptical circulator of claim 10 wherein said second tapered plate offsetslight traveling from said second port by a distance f′ perpendicular tosaid longitudinal axis.
 16. The optical circulator of claim 15 whereinsaid distance f′ is in the plane of said angle φ.
 17. The opticalcirculator of claim 10 wherein said angle θ lies in a first plane andsaid angle φ lies in a second plane parallel to said first plane. 18.The optical circulator of claim 17 wherein said mutually orthogonalpolarizations and said mutually parallel polarizations are separated andcombined in said first block of birefringent material and in said secondblock of birefringent material in a third plane perpendicular to saidfirst plane and said second plane.
 19. The optical circulator of claim10 wherein said first compound polarization rotator comprises first andsecond reciprocal polarization rotators and a first non-reciprocalpolarization rotator; and said second compound polarization rotatorcomprises third and fourth reciprocal polarization rotators and a secondnon-reciprocal polarization rotator.
 20. The optical circulator of claim10 wherein said first compound polarization rotator comprises first andsecond non-reciprocal polarization rotators, and said second compoundpolarization rotator comprises third and fourth non-reciprocalpolarization rotators.